Without limiting the scope of the invention, its background is described in connection with systems for water purification, in particular, for desalination. The membranes are commonly used in purification of water whether it is surface water (rivers and lakes), or underground water (aquifers) or industrial and municipal wastewater (mining, metals, dyeing, chemical) or produced water from oil and gas extraction or salt removal from seawater or brackish water.
Membranes are very efficient and energy friendly for water purification applications. They are engineered and highly optimized to remove specific material from the feed effluent, for example, total dissolved solids (TDS) in form of mono and divalent ions such as NaCl, MgSO4, CaSO4, or other high molecular weight materials such as sugars, chemicals, organic molecules and other macromolecules. An RO membrane is optimized to reject Na+ Cl− ions for effective desalination while UF and MF membranes are designed to reject submicron particles or higher molecular weight impurities and remain permeable to dissolved salts.
Desalination is a combination step of several processes that remove excess salt and other impurities from water. After desalination, seawater, brackish water or surface water can be converted to fresh water suitable for human consumption and irrigation. Currently, more than 1,300 desalination plants are operational in the United States, producing over ½ billion gallons of water per day. Today's interests in desalination are focused on developing cost-effective ways to provide fresh water in the regions where availability of fresh water is, or is becoming limited.
During the last couple decades, membrane desalination and water treatment processes have developed very quickly, and most new facilities employ reverse osmosis (RO) technology. The RO process uses semi-permeable membranes and pressure to separate salts and other impurities from water. The membranes used for RO have a polymer matrix with a dense barrier layer where most separation occurs. They are able to treat water with varying salt and impurity concentrations over a wide range of 50 to 50,000 parts per million (ppm, also referred as milligrams per liter) of total dissolved solids (TDS). RO plants consume about 30 Watt-hours of electricity per gallon of (seawater) water desalinated or less depending on the salt content concentration. In most cases, the membrane is designed to allow only water to pass through this dense layer, while preventing the passage of solutes such as salt ions. These membranes are capable of removing up to 99.5% of salts, particulates, dissolved organics and emulsified oil. The process requires that a high pressure be exerted on the feed side of the membrane, usually 50-300 psi for surface and brackish water, and 500-1,000 psi for seawater, to overcome the osmotic pressure corresponding to the salt concentration. RO membrane systems typically use less energy than other desalination techniques such as thermal distillation, ion exchange, and electro-dialysis, and have led to a reduction in overall desalination costs over the past decade.
Today, including capital expenditure for desalination equipment when depreciated over time, operational expenditure for energy, membrane, cleaning chemicals, system maintenance and labor, costs for brackish water and seawater RO plants are approximately $1/1000 gallon and $3/1000 gallon product, respectively.
Continued efforts to reduce the cost of desalination have driven a large number of technological advances. These include evolution of advanced pre-treatment MF/UF membranes with wide range of materials and geometries. Hollow-fiber membranes (HFMs) have evolved as the geometry of choice due to their ease of cleaning and regeneration after fouling. Both inside-out and outside-in filtration geometries exist in HFMs. In addition to two geometries, several membrane material compositions such as PVDF, PAN and PES are also available. Due to large variations in geometries and materials, membrane manufacturers are able to lock in relatively high prices for proprietary designs and materials for fixed process flow equipment. Prices can vary significantly, anywhere from $3/ft2 to $10/ft2 of membrane surface area; however, performance differences are very limited. Membranes that are commonly available and can be produced by several companies tend to be lower in prices, while configurations and materials variations may still exist. End users generally get locked-in with a fixed system design and have limited flexibility to choose other membrane types during replacement cycles.
To address such limitations we have come up with a novel process flow and control strategy that allows an open architecture system with flexible pre-treatment UF/MF segment that can accept a variety of membranes from common suppliers including inside-out or outside-in geometries on same platform. The control system is able to accept several commercially available membranes in the market with minimal changes.
In an optimized desalination system, RO membranes are not allowed to be subjected to influent water that has high level of particulates. Measured using silt-density-index (SDI), the feed water must remain below 3 SDI. To achieve such low level of particulates, system manufactures provide one or two steps of microfiltration cartridges (say 30 um followed by 1 um) which are replaced frequently as they get clogged. In turbid waters that are usually found in coastal regions, these filters require frequent replacements and directly impact the performance and uptime of RO.
With access to more affordable MF and UF membranes, that can be backwashed frequently and regenerated periodically when fouled with suspended particles, and are able to last several years, the most cost effective desalination implementations involve use of advanced pre-treatment using MF and UF membranes as discussed above.
Unfortunately, due to complexity of MF and UF operation and maintenance, particularly related to their backwash, chemical regeneration, and periodic maintenance, systems developers usually offer them as individual components in the system. This significantly adds to the overall costs of the pretreatment since control system components are doubled (one for UF, one for RO), the labor to operate and maintain two separate system components is also doubled, and the resources required to build two separate control strategies and related software, middle interface adds to the cost and complexity of the system.
To reduce cost significant and simplify the operation of desalination with a UF/MF pretreatment, we developed an integrated UF/MF/RO system with single control system architecture while providing an operational scheme that further support open architecture on membranes and allow scaling of this approach from small scale skid mounted systems to mid- or large-scale commercial systems.
It would be very important to discuss some quantitative facts that the UF/MF filtration systems require periodic backwashes at high-flux rate, typically about 4-5 times (x) rate compared to the rate of normal filtration. Typically a UF/MF membranes will produce permeate for 20-30 minutes continuously at a pre-specified rate of filtration, followed by a 30 seconds to 1 minute of back-flush using filtrate water at about 4× the forward filtration rate. This results in overall production efficiency of the UF/RO component in the range of 80-85%.
It is worth noting that in a typical seawater desalination scenario, the overall efficiency of small-size (few thousand gallons per day) skid mounted RO systems is limited to about 30% while it can reach to 40% for mid-size (few hundred thousand gallons per day) systems. This means the overall productivity for a pretreatment UF/MF component must be sized to 2.5-4× the productivity of the RO component for a two stage desalination approach. For this scenario, any amount of improvement in overall UF/MF efficiency makes a significant impact on the footprint and the overall costs of the system. In addition, improved efficiency of UF/MF process allows operation of these membranes at reduced flux that further reduces the fouling on the membranes resulting in additional benefits for less cleaning and maintenance.
Using a novel process flow, we improve the overall efficiency for the UF/MF membranes to nearly 97% without any sacrifice in their performance. This is feasible with the integrated MF/UF/RO system architecture where we utilize the final reject water from RO (also called RO concentrated) for the back-flush of the UF/MF membranes.
Since all of the water going into RO subcomponent is already filtered through UF/MF subcomponent, it is clean of any particulates that may tend to foul the UF. During the RO pressurized separation, only TDS concentration is increased in the RO reject, thus it is not expected to cause UF/MF fouling during the back-flush. We generally have to only be careful with possible scaling from scalants present in the water and as long as pH is not in favor of scale producing conditions. In conditions, where scaling risk exists, during back-flush, small quantity of pH reducing acid can be injected. With nearly 10% increased productivity, it translates to nearly 25-40% reduction in the footprint of the pretreatment component in fully integrated UF/RO system.
In addition to UF/MF pretreatment, the presence of large suspended particles and heavy soil/metal, sand particles in the influent, commercial vendors will install additional filtration components such as disk-filtration, sock-filtration, screen filtration, etc. We implemented a novel two-step sequence of large-particulate filtration that combines a centrifugal filter followed by a removable screen filter to address both heavy and light floating greater particulates. Our strategy allow settling of heavy particulates before it enters a screen filter and light floating particulate to separate in screen filters. This combo strategy protects UF/MF membranes from abrasive particulates and allows very easy maintenance of screen filter. The combination strategy, never been shown before, significantly reduces the footprint required to clean water that meets the influent requirements of UF/MF membranes when compared to disk filtration or other screen filters.